In general, there are two types of seismic exploration methods in which seismic energy is injected into the earth at a first surface location and upon traveling through the subsurface formations is detected at a second surface location. In one such method, the reflection method, seismic energy is directly reflected by the boundaries between the subsurface formations and returns to the earth's surface. In the other of such methods, the refraction method, the seismic energy meets such boundaries between subsurface formations at such an angle that it is refracted along a path that passes through the lower of two formations substantially parallel to the boundary between the formations. Upon emerging from the lower formation into the upper formation, the seismic energy is again refracted at the boundary with a similar angle to that at which it originally entered the lower formation from the upper formation. Upon returning to the surface, the seismic energy is recorded as seismic refraction signals.
Refraction seismometry is of course nowhere near as popular as reflection seismometry. The only modern book which considers refraction prospecting in considerable depth is the volume "Seismic Refraction Prospecting," published in 1967 by the Society of Exploration Geophysicists under the editing of A. W. Musgrave.
In refraction seismic exploration, a number of seismic detectors are usually, but not necessarily, spaced at greater distances from one another than in reflection shooting, with the refraction detector array being spaced at great distances from the source of seismic disturbances, in practice as much as 5 to 15 miles. Thus, refraction seismic exploration enables large areas to be quickly surveyed, and also enables mapping of certain remote areas in which reflection shooting would be extremely difficult or costly. Further, in refraction exploration, refracted signals having relatively low-frequency spectrums, and therefor low attenuation, are of interest as compared to the higher frequency spectrums often of primary interest in reflection shooting. The use of refraction shooting is also desirable in mapping massive geologic members such as limestone layers or the like, as the velocity information provided by such refraction techniques assists in correlating and identifying desired events or key horizons.
Improved techniques have been developed for obtaining and interpreting refraction data, a number of which are described in Seismic Refraction Prospecting, published by The Society of Exploration Geophysicists in 1967. However, in spite of the many advantages which attach to refraction seismic exploration, the use of reflection techniques is currently more common. It is believed that one reason for this situation is that suitable techniques have not been heretofore developed for suppressing unwanted events and enhancing desired events in seismic refraction work.
In oil and gas exploration, seismic reflection shooting has been well known and practiced for decades. Since the mid 60's, common-depth-point (CDP) recording of seismic reflection data has been the major surface exploration technique for oil and gas reserves. After its introduction (Mayne, 1962) it took only a few years for geophysicists to realize the fundamental properties of CDP stacking. Mayne's original concept was that of large receiver arrays simulated by CDP stacking without reflection point smearing. This concept has proven itself time and time again as the best reflection seismic data enhancement technique available. Today, the term common-mid-point (CMP) stacking is commonly used rather than CDP since it better describes the geometry of the method. CMP seismic traces are all those traces which have the same geometrical mid-point half way between their corresponding source location and receiver location.
Seismic data is typically collected with one source or source array being recorded into many receivers or receiver arrays. As many as 480 receiver arrays are actively recording data from a single source. Receiver arrays are generally spaced at even increments along the seismic line of traverse. The distance between the source and any given receiver array is normally referred to as source-to-receiver offset or simply offset. To achieve CMP geometry, the active receiver arrays are incremented as the location of the source moves along the seismic line of traverse The location of the source is normally incremented at an even receiver array spacing so that the active receiver arrays may be simply "rolled along" at even increments as source locations change, keeping constant offset geometries with the source. The active receiver arrays may be located entirely on one side of the source (end-on), half on each side of the source (split-spread), or somewhere in between (asymmetric split-spread). The CDP fold coverage or number of CMP traces at a given mid-point is controlled by the number of active receiver groups and the source increment along the line.
Before CMP seismic reflection traces are summed (or stacked) together to form a single stacked trace at each mid-point location along the seismic line, various geophysical processing steps would normally be applied to the data. These might include gain, spherical divergence correction, deconvolution, static corrections, normal moveout (NMO) removal, and trace muting. Since traditional CMP stacking is done to enhance reflection signals, the NMO correction applied is hyperbolic in source-to-receiver offset. In areas of considerable structural dip in the subsurface, a dip dependent hyperbolic NMO might be used. The amount of hyperbolic moveout is determined by the NMO or stacking velocity. Stacking velocity is a function of root-mean-square (rms) velocity and structural dip. In the presence of dip, the stacking velocities are modified by the cosine of the angle of dip. Dip dependent NMO makes this cosine correction and partially migrates reflection energy to true common-reflection points for stacking. Just prior to CMP stacking, data at far offsets and shallow two-way travel times are generally muted or zeroed. This is done to reduce the effects of NMO stretch and to suppress contamination from unwanted refraction arrivals.
A stacked seismic record section is a collection of stacked CMP seismic traces displayed side-by-side in monotonically increasing CMP location. Subsurface structural interpretations can be made from these stacked sections. However, the stacked data is generally migrated to produce better seismic images before structural interpretation. Stacked sections can be generated in several forms. A first method is to generate multiple stacked seismic record sections, each with a different effective stacking velocity. These are called constant velocity (CV) stacked sections or panels. These CV stacked panels can be used to make a velocity interpretation which is needed to form a composite or single final stacked section. The interpreter simply selects certain reflection signals from the multiple CV stacked panels. He then applies the corresponding NMO velocities in a time and space varying manner, with interpolation between panels, to produce a composite stacked section of all desirable reflection signals. The final CMP stacked section will thus use variable velocity functions in both two-way travel time and CMP location. This composite stack can then be used for structural interpretation or passed on to a migration step prior to interpretation.
As previously mentioned, refraction signals are generally muted out in the traditional CMP stacking method. Refraction signals are considered undesirable coherent noise and many efforts are made to suppress this form of energy in the CMP stacking process. In many geographical areas of the world, these refracted signals and other related source generated coherent noise completely mask any reflected signals. These areas are considered no record (NR) data areas and as such offer very little subsurface structural information. However, refraction signals can offer some subsurface structural information. Prior to the introduction of the CMP reflection stacking method, seismic refraction methods were widely used to map subsurface structure. Indeed, many of the large oil reserves found in the first half of this century were found using refraction methods. Today, the use of refraction signals is confined mainly to computing near surface static corrections.
Reflection events will be generated at all acoustic interfaces in the subsurface. However, refraction events will only be generated in high velocity layers which underlie lower velocity layers, and then only if certain other conditions are right. Reflection events will occur on both the nearest offset seismic traces and the farthest offset seismic traces, whereas refraction events will only occur on seismic traces which are recorded beyond the critical offset distance for a given refraction interface. To record refraction events from deeper and deeper acoustic interfaces, one must record longer and longer source-to-receiver offset traces. A general "rule of thumb" is that one needs offsets which are about three times the vertical depths of interest in the subsurface. Typical maximum offsets used today in reflection CMP recording are on the order of 3000 meters. Thus, refraction events might be expected to be present on these data down to depths of about 1000 meters.
For planer refraction interfaces, refraction events will have linear moveout with increasing offset. This linear moveout will be a function of structural dip and the refractor velocity. In a shot profile gather of seismic traces, the linear moveout for a given planer refractor will be different in the up-dip direction from the down-dip direction (split spread recording). However, in a CMP gathering of seismic traces, there is no difference between up-dip and down-dip moveouts because of source/receiver reciprocity. In a CMP trace gather, the linear moveout velocity of a refraction event from a single planer cosine of the angle of dip in the refractor.
Prior work with seismic refraction data has not included CMP stacking of the refraction data U.S Pat. No. 3,629,798 (D. W. Rockwell, 1971) worked with refraction data but only stacked data from a single shot. No data was gathered over a CMP for stacking, to provide the advantage of properly imaging refraction wave arrivals.
Adams et al received U.S. Pat. No. 4,232,378 in 1980, which relates to a refraction seismic technique, which studies the amplitude of long and short shot-to-receiver-distance refracted waves. There is no discussion of stacking the refracted waves.
Also, Ruehle teaches a technique for acquiring refraction data in U.S. Pat. No. 4,242,740, but does not disclose a method of stacking the data.
Gassaway et al. received related U.S. Pat. Nos. 4,373,197; 4,393,488; and 4,528,649 which were assigned to applicant's assignee. A `roll-along` technique of shifting source and detector arrays is disclosed, whereby the resulting refracted data can be systematically indexed to offset position. Overlapping stackable displays are produced which are indexed to a common inline position and to refraction travel direction. However, no CMP stacking technique is disclosed, and the method instead relates to distinguish shear wave data from compressional wave data.
Monastyrev, V. K. et al received Union of Soviet Socialist Republics Patent No. 864215 in 1981. Multiple profiles of refracted waves are recorded, to also record elastic oscillations at known distances from seismic excitation points, which are nearly the same as the distances to the initial points where refracted waves exit at the surface. The method provides for multiple tracking of common depth refracting areas. Refracted waves corresponding to this total depth area are selected and tau-P summed using various cutoff velocities, which are determined according to the maximum energy values and signal-to-noise ratio. There is no discussion of the need to properly mute the arriving waves which originate at a point inside the critical offset distance. Nor is there a discussion of the use of several constant velocities to generate multiple panels of summed data. Also, Monastyrev et al do not discuss using a generated near-surface velocity model to create a solution to near-surface statics.
A Russian brochure, whose title has been translated as "Method For Studying Refracting Boundaries In Geologic Layers" is dated Sept. 5, 1988. It is believed that the brochure was published by the Western Siberia Geophysics Institute which is part of the U.S S.R. Ministry of Geology. The brochure may be further identified as RD 03345, and print order 1024. It relates to a common depth area seismic refraction method, which is based on multiple summation of useful information in the refracted waves. The method itself is not disclosed, and only the benefits of the method are described. Disclosed advantages include simultaneous investigation of several geologic boundaries, being able to study geologic refracting boundaries, and being able to detect heterogeneities in the near-surface section field data.
Hinkley (U.S. Pat. No. 4,577,298) discloses a method estimating and correcting source and receiver statics contained in seismic traces. Refraction ray paths are merely normalized to the paths taken by the reflection components to correct for angular displacements between the refractions and reflection signal components.
Yang, H. published an article entitled "Stacking and Migration Technique For Seismic Refraction" in December 1986. However, neither CMP nor CDP stacking were discussed. Instead, Yang only utilizes a method of common receiver or common source point stacking.
The above methods are all limited in the attempts to image seismic data in that no attempt has been made to incorporate CDP or CMP stacking of the refraction data as taught in the subject application. To date, refraction arrivals are primarily used to generate near surface velocity models for statics computations. Once `first break` picks on refraction events are made, the seismic data is discarded. No attempt has yet been made to produce an image of the refractors themselves.
Current methods of utilizing only reflection data frequently fall short of providing adequate seismic images of the geology below the earth's surface. There is therefore a need for an improved seismic method to obtain better quality seismic data.